Core Shaper Parametric Eq [documentation]

[dylan159]

I made a PCB for this circuit. It works and sounds good. I made the PCB small to fit in a guitar and replaced all but the mid pots with trim pots. I am really pleased with it but I have a couple of issues I need to resolve.

I tried to use an on off switch on the power but this is really noisy so instead of turning the power on and off, I am using a switch to turn the Led on and at the same time switch my signal output from the pedal input to pedal output. This is quiet during switching but is there a more sensible way to bypass the circuit or switch it into my signal path.

The other issue which might be related but I am not sure, is that I can get a very high frequency hum. It is like a whistling sound and the dog hates it!

Any thoughts as to how this might be created and how I might eliminate it?

Thanks
Nick

Surely not the first use I'd have thought of (especially with the trim pots, but this is a bit more involved than the standard 3-band baxandall present in many basses), but it goes to show how versatile it can be!
Switching power is not a good idea, as you found out. Current draw shouldn't be too big of an issue, especially if you change the 5532 for something more frugal and forgo the LED, so what I'd do is use a DPDT, either toggle or push-pull, and true bypass the circuit. The switch position is indication enough I think, and you don't have the additional 1M load on your signal even when off.
For power, you can otherwise make a box with 9v and TRS to TS passthrough, which sends the positive supply to the ring of the cable and you can get it back inside the guitar. It's an idea I had for a while and the best thing about it is that if you have the bypass switch, you can use it as a normal passive guitar with any TS cable.

The whine could indeed be the charge pump, you can check if it's exactly 10kHz with a scope or a phone app. While shielding and additional RC filtering might work, you should use the charge pump in boost mode at 45kHz, which won't give any of these problems. Be sure the one you're using support this frequency.

[NickH]

The circuit works really nicely on a guitar. I take your point regarding keeping the power consumption down. I have used a stereo socket for the input Jack which means that the battery is disconnected when the jack is removed but during playing, power is supplied to the PCB but the LED is at least turned off which will lower consumption a bit. I suppose I will have to see how it behaves over a period of weeks/months as to whether that approach is feasible. If you are interested, you can see the guitar I made that contains the circuit here:

https://www.instagram.com/reel/ChngUOEj ... MyMTA2M2Y=

Thanks for the various replies regarding the high frequency hum. I may have realised another reason for the issue in my situation. There appears to be a shortage of LTC1044 8-SOIC chips. Whilst I used this on my prototype versions, I ended up replacing it with a MAX1044 chip which is meant to be a pin-compatible alternative. I have struggled to understand the data sheets regarding the switching frequency but the purpose of the boost is to increase the frequency and move it out of range for audio applications. I am thinking that the MAX1044 chip may be operating at a lower overall frequency than the LTC1044 and even with the boost, it is producing noise that is then amplified by the eq circuit.

I still have to analyse the frequency of the noise but will do this shortly and fortunately I still have a breadboard version that uses the LTC1044 for comparison. DIP versions of the LTC1044 are still available.

I have found a possible alternative which is TC7660S. This supposedly runs at 45KHz when boost is selected. Sadly, I can't find a SOIC version of this but may have to use an adaptor to put a DIP version onto the SOIC pads of the PCB.

Would appreciate your thoughts on this as a possible explanation for the the high frequency oscillation.

[mauman]

Regarding charge pumps, the specs for slightly different part numbers can vary a lot, even from the same manufacturer. The Microchip TC7660S boosts the switching frequency from 10 kHz to 45 kHz if you connect pin 1 to V+, and tolerates up to +13V in. The Microchip TC7660H runs at 120kHz, pin 1 has no function, and anything over 10.5V in will fry it. Variances are also large across different manufacturers using the same part number, the Microchip 7660S and the ICL 7660S have different specs. And unfortunately, the data sheets can be vague about important things like maximum output current.

[dylan159]

If you are interested, you can see the guitar I made that contains the circuit here:

https://www.instagram.com/reel/ChngUOEj ... MyMTA2M2Y=

Nice video and nice guitar. You could measure current draw directly and make an estimate based on battery capacity, maybe cutting it by half if you don't want the voltage to drop too much.
Please don't call it hum if it's high frequency, I'd call hum something below 200 Hz, even if not exactly mains frequency.

Regarding charge pumps, the specs for slightly different part numbers can vary a lot, even from the same manufacturer. The Microchip TC7660S boosts the switching frequency from 10 kHz to 45 kHz if you connect pin 1 to V+, and tolerates up to +13V in. The Microchip TC7660H runs at 120kHz, pin 1 has no function, and anything over 10.5V in will fry it. Variances are also large across different manufacturers using the same part number, the Microchip 7660S and the ICL 7660S have different specs. And unfortunately, the data sheets can be vague about important things like maximum output current.

In addition to this, MAX1044 has a nominal frequency of 5khz without boost at 5v and the datasheet says boost makes that six times more which should be enough. The 7660 without the S doesn't have a functional boost pin instead.

[NickH]

Thanks again for the replies. I have measured the switching noise on a circuit with a Max1044 used. This was the only surface mount version of any of these regulators I could find. Even though pin 1 is tied to V+ and so should be operating with boost, the frequency I measure is around 9KHz. This is less than I would expect but puts it into the audible range. I can try and shield the circuit as per a previous suggestion but would not wish to do that in the long run. I have yet to go and check the same circuit but where I used a DIP version of the chip specified on the original drawing. If that is operating at a higher frequency out of the audible range, then the issue is with the Max1044.

It seems that my options are to try again to find a SOIC version of a chip that is compatible but operates at a higher frequency such as the TC7660S, or to get an adaptor that allows me to put a DIP type chip on a SOIC footprint. The DIP versions are still available.

That said, getting a batch of new PCBs made with DIP footprints would probably be cheaper than buying the adaptors and at the same time allow me to correct a couple of mistakes on the board that I had to fix with a couple of jumper wires.

[dylan159]

That said, getting a batch of new PCBs made with DIP footprints would probably be cheaper than buying the adaptors and at the same time allow me to correct a couple of mistakes on the board that I had to fix with a couple of jumper wires.

how many of those boards to you need? A quick solution would be an RC filter on the negative rail: 9khz is easy to filter.
this might be the first time I hear someone being forced to use tht, usually it's the other way

[NickH]

I think there is a global shortage of some electronic items. Some chips like the one in the schematic are on back order, not due until 2023. I actually found a suitable SOIC footprint chip that I have ordered and will try out. If that doesn't work, then I will put in a filter.
I have made about 5 guitars that can accept this PCB but because I had them made in china, it was very cheap to order 50 so that would keep me going for a really long time. So I don't really want to do this again but since it only costs about £40.00 ($50.00) to have them made, it is possibly easier to redo them than modify what I have already. Another advantage of version 2 is that I can probably make the board a bit smaller and reconsider the layout.
I'll keep this chat updated as to how I get on as others may find the use of a different chip of use.

[NickH]

OK, I can report back with some good news that may be of use to others. I did a direct replacement of the MAX1044 SOIC footprint chip that I used because I could not get hold of the original LTC1044. The Max1044 was giving out 9.2KHz high frequency switching noise and this is within the audible range and so whilst not so noticeable for deaf old gits like me, it was bothering the dog and the kids.
I found a SOIC replacement TC7660H which runs at 120KHz at the suggestion of mauman. It does not have the boost function but because pin 1 is not connected, it can be a direct replacement in my circuit or if the boost is not used because it runs at high frequency anyway. So no need to boost it.
So, a bit of a lesson in reading and understanding the data sheets for chips and just because it says pin compatible, doesn't necessarily mean it is a good replacement. In this case, the lower switching frequency just enters the audio range and makes it a poor choice.
To test it out, I asked my 18 year old son and he couldn't hear a whistling noise and the dog didn't leave the room so I think that is a proper scientific test completed.
Thanks all,
Nick